The invention relates to a method for casting metallic objects in a casting device for precision casting with at least one casting mold, one device for pouring liquid metal into the casting mold and with devices for generating vacuum and pressure and a casting device for carrying out this method.
By the term precision casting is understood the casting of jewelry, objects of art or decoration, as well as of precision hardware for the industry, in particular comprising gold, silver, platinum, bronze and other metals. Casting devices are known in which a melting crucible and a casting mold are disposed in a receptacle. The melting crucible is equipped with an inner cavity for receiving raw material and with a heating device, for example an electric induction device. As pouring means can be used the known possibilities with the known example comprising a bottom drainage with a stopper. Underneath the melting crucible is positioned the casting mold which comprises a gas-permeable porous material. The casting mold has a mold cavity which most often makes possible the casting of a multiplicity of discrete parts in the same casting step, i.e. the mold cavity has a tree structure with an inlet funnel. The production of the casting mold is most often carried out with the aid of a model comprising a synthetic material or wax and the mold can only be used once. The method for casting metallic objects in this known casting device is comprised of several steps. First, with the receptacle open the melting crucible is filled with raw material and the casting mold in set into the lower part of the receptacle. The lower part of the receptacle is separated from the upper part.
The upper receptacle part is connected to a device for generating excess pressure and the lower receptacle part with a device for generating a vacuum. To initiate the casting process the receptacle is closed gas-tight and the melting process is initiated in the melting crucible. By opening the stopper the liquid melt flows by casting from the top into the mold cavity of the casting mold until it is completely filled. Before and/or during the casting process the lower receptacle part in which the casting mold is disposed, is exposed to a negative pressure which also extends into the mold cavity due to the porosity of the casting mold material. At the end of the casting process, i.e. when the mold cavity is filled, an excess pressure is generated in the upper receptacle chamber such that this pressure acts also onto the surface level of the melt in the inlet gate of the casting mold. This combination of negative pressure acting onto the bottom and the shell of the casting mold and of excess pressure acting onto the melt in the mold cavity, compared to casting methods with bilaterally equal pressure, leads to better filling of the mold cavity and better molding of fine detail.
In spite of the casting results, which per se were good, with this known device, problems occur repeatedly, in particular in the case of complicated and delicate casting objects. The liquid melt can, for example, be distributed too slowly in the mold cavity. The consequence is that to some extent fine complicated branchings are not filled out or differing structures of the solidified metal occur since the cooling rate and the of solidification time are different in differing regions of the mold cavity.